Miniaturized contact spring

ABSTRACT

This invention provides a solution to increase the yield strength and fatigue strength of miniaturized springs, which can be fabricated in arrays with ultra-small pitches. It also discloses a solution to minimize adhesion of the contact pad materials to the spring tips upon repeated contacts without affecting the reliability of the miniaturized springs. In addition, the invention also presents a method to fabricate the springs that allow passage of relatively higher current without significantly degrading their lifetime.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of U.S. Provisional PatentApplication Serial No. 60/365,265 filed Mar. 18, 2002, and is relatedU.S. patent application Ser. No. 09/979,551 filed May 26, 2000; Ser. No.09/980,040 filed Jul. 28, 2000; Ser. No. 10/094,370 filed Mar. 8, 2002;Ser. No. 10/069,902 filed Nov. 21, 2002 and International PatentApplication PCT/US02/26785 filed on Aug. 23, 2002

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] This invention relates generally to the highly miniaturizedsprings. More particularly, this invention relates to a family ofminiaturized contact springs and a family of methods for increasing theyield strength and fatigue strength of these springs.

[0004] 2. Description of the Prior Art

[0005] Miniaturized springs have been widely used as electrical contactsto contact pads or I/O terminals on integrated circuits, PCB-s,interposers, space transformers and probe chips for purposes such astesting, burn-in and packaging because even arrays of such miniaturizedsprings can be fabricated with a pitch of less than 10 μm.

[0006] A miniaturized stress metal film spring, usually patterned byphotolithography, comprises a fixed portion, also called anchor portion,attached to a substrate and a lifted portion, also called free portion,initially attached to the substrate, which upon release extends awayfrom the substrate forming a three dimensional structure as a result ofinherent stress gradient in the spring. Typically, the stress gradientin a film is produced through sequential deposition of a plurality ofthin film layers by sputtering or electroplating under different processconditions. A typical embodiment of stress metal spring is schematicallyshown in FIG. 1a, which comprises an anchor portion 101 associated withan electrical contact or terminal 102 attached to a substrate orelectrical component 103, and a free portion 104 with a spring tip 105.Examples of such structures are disclosed in U.S. Pat. No. 5,613,861(Smith) and application PCT/US00/21012 (Chong, Mok).

[0007] Other types of springs include discrete springs, fabricatedindividually or in a group and subsequently mounted on a substrate, suchas those used in wafer test or burn-in assembly, or those comprisingintegrated solid-state devices such as semiconductor devices. Stillother types of such springs are those cantilever types of springs, whichare fabricated en masse on a substrate using photolithography, asmentioned in the patent literature such as PCT 01/48818, PCT WO97/44676,U.S. Pat. No. 6,184,053, and PCT WO01/09952. Some of these springs arefabricated individually or in a group on a sacrificial substrate andthen mounted onto substrates used in the wafer test or burn-inassemblies, or onto those comprising semiconductor devices. FIG. 1b is aschematic cross view of a typical photo-lithographically patternedfreestanding cantilever spring fabricated on sacrificial layers, whichcomprises a base region 201 at one end that is attached to an electricalcontact pad 202 of a substrate 203, a contact tip region 204 at theother end of the spring, and the central main body 205 of the springconnecting the base 201 and the contact tip region 204. The problem inthis kind of springs is that they are too long. Shorter and smallersprings are desirable for testing and burn-in of some of the current andnext generations of integrated circuits, which comprise contact terminalpads with very small pitch, 20-50 μm, for example.

[0008] Methods of fabricating shorter springs using photolithographically processes to add thicker metal coatings have beendefined in the patent literature. One method is described in applicationWO 01/48870. This method uses electroplated photo resist to allow metalto be plated on the top of a free standing spring. However, at thedimensions needed to probe ICs with pad pitches below 150 μm thefreestanding springs have insufficient strength to hold backside photoresist without significantly reducing the probe height required forcompliance. Any non-uniformity in the photo process also translates tonon-uniform spring heights that cannot meet the uniformity requirementsnecessary to stay on the IC pads while testing.

[0009] The method described in application (WO 01/48870)) also has anadditional problem in controlling lift height after plating. One of thepurposes of having a freestanding spring is to provide a framework orstructure to support the thicker plated metal. If one plates a spring ononly one side, the spring curves to a different lift height based on thestress in the plated film. If the film is tensile it curves up and if itis compressive it pushes it down. Both of these stress conditions aredifficult to control for the tolerances and spring lift uniformityneeded to test ICs. In addition, compressive springs are stronger thantensile springs and the spring with a compressive plated film loses liftheight to the point that there is not enough compliance for it to stillbe a useful probe. There is also a limit to how high a freestandingspring can be lifted prior to plating to compensate for this compressioneffect. The probe needs to make contact to the IC electrical pad at anangle less than 90 degrees. Increasing the lift height tends to causethe spring to wrap around itself creating a 360 degree circle to thesubstrate. As a result, the process taught by this patent applicationdoes not meet the requirements for controlling uniformity of the liftheight of arrays of springs required for IC testing.

[0010] One method to build the probe in application WO 01/48870 is toassemble a tip on the plated spring and assemble the spring fabricatedon a sacrificial substrate to a second interconnect substrate. Theassembly process adds positional placement errors and is more expensiveto manufacture than a fully integrated connection button tip asdescribed in the invention herein.

[0011] Another method described in patent number U.S. Pat. No. 6,528,350keeps the photoresist coating, i.e. mask, off the spring, and uses arelease layer island to allow plating of the freestanding portion of thespring. For cases where the release mask stops adjacent to the base(anchor portion) of the spring and does not extend along the base of thespring, the thickness and width of the free portion of the spring closeto the base become much larger upon plating compared to the base region.As a result, the freestanding portion of the spring is mechanicallyweaker in the vicinity of the base region. Because the bending moment isthe highest in this region, upon application of a force to the springtip during IC test, the springs fracture early and therefore can notmeet the probe lifetime requirement needed in IC manufacturing lines.For the other method described in U.S. Pat. No. 6,528,350, where thephotoresist mask does not cover the freestanding portion of the springas well as part of the anchor portion during plating, there still is adiscontinuity in the width which tends to fracture.

[0012] The mask alignment and control of spring release process alsopose serious problems resulting in uneven plating and a variation inlift uniformity. Another major problem in this process arises from thehigh resistivity of the relatively thin release layer, as well as thestress metal film, through which the plating current flows. The currentdensity varies widely with the distance from the power connection pointsat the edge of the substrate. As a result, the plated filmcharacteristics, e.g. microstructure, thickness, stresses etc., varywidely at different areas of the spring. As a result, this process doesnot produce arrays of springs with reasonably uniform and controlledproperties, such as lift height that is essential for effective ICtesting.

[0013] The invention herein comprises several means to circumvent theproblems associated with the above two methods and provides solutionsthat allow manufacturing of arrays of springs suitable for meeting thestringent requirements of wafer level IC testing. Among other things,the invention allows fabrication of arrays of springs with reasonablyuniform lift height and properties, as well as durability. For example,it teaches the practice of enveloping the entire spring core, bothfreestanding and anchor portions, with electrodeposited films with abalanced stress that allows maintenance of spring heights withappropriate uniformity after electrodeposition. In another teaching, itshows a method to plate the springs selectively without the use of anyphotoresist mask.

[0014] The miniaturized contact springs are subjected to a large numberof contact operations during testing which subject the springs tovarious levels of stresses including cyclic stresses. Also, in packagesthat use contact springs to join two components, such as chips and chipcarriers, the springs are subjected to stresses during testing andoperation. The springs are required to withstand such stresses withoutfailure. However, we have observed that the miniaturized springs, suchas those with a size of around 400 μm×60 μm×20 μm, start to fail, i.e.being plastically deformed and/or fractured, typically after 10,000touchdowns, where the contact force exceeds about 1 gf. A major reasonof the failure is that the resulting alternating stresses exceed fatiguestrength of the spring material. The fatigue strength indicates thealternating stress level at which a material can withstand a specifiednumber of cycles. It is typically some fraction of a material's yieldstrength, which corresponds to the onset of plastic deformation, i.e.instantaneous permanent deformation. Because a force exceeding about 1gf is usually required to make good reproducible contacts on aluminumwith low contact resistance, as observed in our experiment, theresistance of the springs to failure must therefore be significantlyincreased to improve the performance and quality of the springs. Springswith larger cross-sections can withstand similar or larger force withoutfailure because the resulting stresses are lower, but they limit thepitch at which springs can be built.

[0015] For some operations, such as the burn-in of devices, contactsprings are required to make contacts with the device terminals at anelevated temperature, for example around 100-C. Such contacts may alsobe required to allow passage of a relatively high current, for example250-500 mA, during the operation. Under this condition the contactresistance should be quite low, for example 0.1 milli-ohm, so that thecontact tip region of the spring may not get damaged by overheating. Oneway to achieve the low contact resistance is to increase the contactforce, by increasing the thickness of the springs. However, a highercontact force increases the stress developed in the body of the spring,particularly near the base region, and thus increases the probability ofearly spring failure during repeated touchdowns.

[0016] Furthermore, the materials from electrical contact pads orterminals tend to adhere to the spring tips during repeated contacts. Incases where the adherence of the pad material to the spring tipsincreases the contact resistance, or the pad material readily formstenacious compounds upon exposure to the ambient condition, theelectrical contacts are degraded after repeated touchdowns. This alsoshortens the springs' lifetime. Thus contact tip structure shouldpreferably be comprised of materials that do not have strong adherenceto the contact pads or terminals.

[0017] Therefore, what is desired is a mechanism for maximizing yieldstrength and fatigue strength of the miniaturized springs within theminiaturization requirement.

[0018] What is further desired is a mechanism to minimize adhesion ofthe contact pad materials to the spring tips upon repeated contactswithout substantially affecting reliability and electrical conductivityof the springs.

[0019] A method to fabricate springs with high resistance to compliantstress that results in uniform spring height and provides for a durabletip structure is desired.

SUMMARY OF THE INVENTION

[0020] This invention provides a solution to increase the yield strengthand fatigue strength of miniaturized contact springs, which can befabricated in arrays with ultra-small pitches. It also discloses asolution to minimize adhesion of the contact pad materials to the springtips upon repeated contacts without affecting the reliability of theminiaturized springs. In addition, the invention also presents asolution to fabricate the springs that allow passage of relativelyhigher current without significantly degrading their lifetime. Also, theinvention provides a solution for fabricating robust springs for joiningdie-bonding terminals to corresponding input-output pads of a substratecomprising an inorganic or organic material for reliable packagefabrication. The joining can be facilitated, for example, through theuse of solder or conducting adhesives including anisotropicallyconducting adhesive films.

[0021] The stress metal spring according to this invention comprises amultilayer film structure. The thin films have substantially greateryield strength and fatigue strength than the corresponding bulkmaterials, and thus these springs allow repeated touchdowns duringtesting or burn-in without any significant plastic deformation, if any.

[0022] Deposition of compressively stressed films onto the core films isfound useful to increase the spring lifetime. This also allowsfabrication of stress metal springs capable of applying large force atthe electrical contact pad or terminal.

[0023] Thin films are deposited with graded transitions in composition,either continuous or in fine discrete steps, across an interface betweentwo different materials, so that the elastic modulus increases, ingeneral, monotonically with depth from the spring surface to the springcore. The resulting springs exhibit a significant increase in lifetimeduring repeated touchdowns.

[0024] Suitable materials and/or process are found useful to increasethe interface robustness in the multilayer structures. Materials ofsimilar lattice parameters are preferably used in the adjacent films andamorphous or nano-crystalline films are used as interfaces. An interfacecan be made by either “phasing-in” the materials of two adjacent layersor by using an alloy of the materials of two adjacent layers.

[0025] The thickness of the free portion of the thin film is preferablywithin the range of 4-35 μm, which allows reliable and low electricalcontact resistance between the spring tips and contact pads orelectrical terminals comprised of different materials.

[0026] At least one high thermal conductivity film in the multilayerfilm structure is preferably used for dissipating heat during testing orburning-in at a relatively high current.

[0027] Alteration of the process parameters during deposition of thefilms constituting the spring structure is found useful to enhancespring quality and reliability. For example, thin films comprising thecoatings of the spring core are deposited with suitable microstructuralfeatures, such as ultra-small grain size, e.g. less than 200 nm, forincreasing both the yield and fracture toughness of the spring.

[0028] The force used to make good electrical contacts between thesprings and the contact pads is substantially reduced. A suitable rangeof force for contact on aluminum (Al) is 0.8-10.0 gf. For the contactpads made of gold, copper or solder the force to make good electricalcontacts is much smaller. The low force photolithographically patternedminiaturized contact springs greatly facilitate the construction ofprobe card assemblies including probeChips, i.e. substrates withattached probe springs for making contacts with IC terminals,interposers and assembly fixtures for testing and burn-in, as well aspackaging, The assembly is greatly simplified by the use of these lowforce springs, as bending, warping and alignment problems are minimized.

[0029] The method to improve the lifetime of the springs includessolutions to minimize surface roughness.

[0030] Variation of spring dimensions such as width and thickness isalso found useful to improve lifetime. In one embodiment, the freeportion of the spring is in a tapered shape.

[0031] The invention also provides a less expensive and effectivesolution to electrodeposit overlying films onto the stress metalsprings, circuit traces and electrical contact pads without using anymask.

[0032] In one embodiment, the spring tip region, also called a buttonedspring tip, is selectively coated using a lithographic process with amaterial that minimizes adhesion of the contact pad materials duringrepeated touchdowns. The thickness of the spring tip region is built upprior to the release of the springs from the substrate.

[0033] In another embodiment, a photoresist is applied and patterned toallow selective coating of the tip region, after the spring is liftedand electroplated as required, with a material that minimizes adhesionof the contact pad materials during repeated touchdowns.

[0034] The solutions for stress metal springs can be applied to othertypes of cantilever springs. One embodiment is applicable to othercantilever springs, with or without a buttoned contact structure at thespring contact tip region. Multilayer films are selected and depositedin a sequence following a specific principle that results in thefabrication of robust high performance springs with high durability andincreased lifetime. The principle requires that the films are to beselected and sequentially deposited in such a way that the elasticmodulus of the outer layers of the springs are lower than that of theinterior layers and there is a progressive increase in the elasticmodulus from the surface layer to the innermost layer of the springs.

[0035] In another embodiment, thin films comprising non-stress metalcantilever springs are deposited with suitable microstructural features,such as ultra-small grain size, e.g. less than 200 nm, for increasingboth the yield and fracture toughness of the spring.

[0036] In still another embodiment, the non-stress metal cantileverspring layer is comprised of at least one deposited film layer with abuilt-in compressive stress.

[0037] Following the same principles, shorter springs with increasedrobustness and higher strength can be fabricated.

[0038] The invention is applicable to testing and burn-in of varioustypes of solid state devices, such as silicon and III-V devices, displaydevices, surface acoustic devices, micro-electromechanical (MEMS)devices.

[0039] In addition, the invention is also applicable to packages inwhich electrical terminals of electronic components are bonded tocorresponding contact pads of an adjacent substrate.

BRIEF DESCRIPTION OF DRAWINGS

[0040]FIG. 1a is a schematic diagram illustrating a typical stress metalfilm spring according to the prior art;

[0041]FIG. 1b is a schematic diagram illustrating a typical cantileverspring according to the prior art;

[0042]FIG. 2 is a diagram illustrating the stress-strain curves of thinfilms v. corresponding bulk materials;

[0043]FIG. 3a and FIG. 3b are schematic diagrams illustrating a stressmetal film spring with a multilayer structure according to theinvention;

[0044]FIG. 4 is a schematic diagram showing a stress metal film springwith a multilayer structure comprising at least one high thermalconductivity film according to one embodiment of the invention;

[0045]FIG. 5 is a schematic diagram illustrating a solution for springdesign and fabrication where a metal filled via, an insulated polymerfilm and an electrical trace are used according to one embodiment of theinvention;

[0046]FIG. 6 is a schematic diagram of a stress metal film spring formedover a via providing electrical connection to the backside of substrateaccording to the invention;

[0047]FIG. 7 is a schematic diagram of a stress metal film spring formedand subsequently plated to improve robustness according to theinvention;

[0048]FIG. 8 is a schematic drawing of a plated stress metal film springcoated by photoresist according to the invention;

[0049]FIG. 9 is a schematic drawing of a plated stress metal springcoated by patterned photoresist, exposing the spring tip according tothe invention;

[0050]FIG. 10 is a schematic drawing of a plated stress metal springwith contact tip material plated on the exposed portion of the tipaccording to the invention;

[0051]FIG. 11 is a schematic drawing of a plated stress metal springwith contact tip material after removal of photoresist according to theinvention;

[0052]FIG. 12a is a schematic diagram showing a stress metal film springwith varying width in tapered shape according to one embodiment of theinvention;

[0053]FIG. 12b is a schematic diagram showing a stress metal film springin tapered shape wherein the tip area is coated with a contact materialaccording to one embodiment of the invention;

[0054]FIG. 13 is a schematic drawing of plated stress metal springs withcontact tip material in an interleaved array according to the invention;and

[0055]FIG. 14a and FIG. 14b are schematic diagrams illustrating twocross sectional views of typical freestanding non-stress metalcantilever springs according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0056] Miniaturized springs can be fabricated using thin film ordiscrete component fabrication technologies such as wire bonding. Ingeneral, for springs to perform satisfactorily in wide range ofapplications, the yield strength of the materials is required to behigher than the stresses applied to the springs during testing orburn-in, or in assembled packages. We have observed that many thin filmstress metal springs are plastically deformed during testing because thespring material's yield strength is lower than the applied stress. Astress metal film typically comprises a strong core film composed ofmaterials, such as molybdenum (Mo) or its alloys, tungsten (W) or itsalloys, with additional overlying film coatings, such as nickel ornickel-cobalt (Ni—Co) alloy films. Some of these films are relativelythick, typically 4×10³ to 10⁴ nm in thickness, which is required inorder to increase the force that needs to be applied by the spring tothe contacting surface for establishing good electrical contact. Highyield strength of such films is required for ensuring satisfactoryperformance of the springs.

[0057] Because the stress metal springs produced by this invention arebatch fabricated, using thin film/IC or MEMS technology, on substratesor electrical components, the springs described herein are particularlysuitable for testing, burning-in and packaging (including 3-dimensionalpackaging and chip to chip-carrier bonding) applications involving probecards, interposers, space transformers, PCB-s, wafers, electroniccomponents and microchips with highly miniaturized contact pads or I/Oterminals with a pitch ranging 3-100 μm. The existing technologies aremostly not suitable for such applications. The dimensions of thecorresponding springs or spring terminals are also very small, typicallyranging from 10 to 1,000 μm in length, 3 to 500 μm in width, and 0.1 to40 μm in thickness. The curl radius of the lifted core is typically20-2,000 μm. Note that the teachings of this invention can be used alsoto produce springs or spring terminals outside of the dimensional andpitch ranges indicated. Also note that the teachings of this inventioncan be applied to both the stress metal springs and any otherminiaturized springs comprising thin films.

[0058] In one preferred embodiment of this invention, multiple layers ofvery thin films, each with a thickness less than about 1.5 to 2 μm, areused to fabricate the springs to increase the yield strength of thinfilm spring materials. This is particularly useful for building up thecoating layers over the thin film spring core. The stress-strain curvesof thin films, shown schematically in FIG. 2, are very different fromthose of the corresponding bulk materials. The materials in thin filmsexhibit much higher yield strength, i.e. the films maintain theirelasticity at a higher stress, and relatively smaller plasticdeformation before failure, as compared to the same materials in bulkform. In general, the thin film yield strength is much closer to thetheoretical strength of the material than the corresponding bulkmaterial yield strength. As the film thickness increases, when greaterthan about 2 μm, for example, the films increasingly exhibit bulkstress-strain characteristics. As a result, the elastic limit of thickerfilms is lower than that of thinner films. In addition, the grain sizeof very thin films is generally much smaller than the relatively thickfilms, which also results in an increase in both yield and fracturestrengths. Thus, the springs with thinner films are more robust.

[0059] In this preferred embodiment a discontinuity in the atomicarrangement is deliberately introduced at the interface between twoadjacent films so that the two films retain their individual mechanicalcharacteristics and the interface impedes defect propagation from onefilm to the other. Altering the deposition parameters after a film isdeposited to the required thickness is one way of achieving this.Another way to engineer this interface is to sequentially deposit twodifferent materials next to each other. This includes use of twodifferent materials, e.g. Cu and Ni with close lattice parameters in twoadjacent layers for enhancing the bonding at the interface. Note thatthis scheme also works if the lattice parameters of the two adjacentlayers are not very close to each other. Using such a scheme, multiplelayers of thin films can be deposited to build up the desired springfilm thickness. The top layer of the spring is preferably a thin filmstructure that resists environmental degradation during storage oroperation and adhesion of the contacting material to the spring surface.The embodiment is schematically shown in FIGS. 3a and 3 b, wherein A, B,C etc. indicate different materials. In illustrating a deposition of thesame material but using different process parameters to form the nextadjacent layer, an asterisk is used to indicate the adjacent layer, e.g.A and A*.

[0060] The multilayer springs formed in this manner can be annealed at arelatively low temperature, if desired, for a short time, e.g. 150-C for10 min to facilitate bonding between adjacent layers and relaxation ofthe internal stresses in the films to impart additional robustness inthe spring.

[0061] In a variation of this embodiment, the deposition condition maybe altered to produce a non-crystalline or crystalline thin film, lessthan about 200 nm, in between two relatively thicker, approximately≦2000 nm, film layers in order to facilitate bonding between adjacentlayers. Examples of such in-between material films are Au, Ag, Ni, Cuand etc.

[0062] Various deposition techniques can be used to deposit themultilayer films, such as physical vapor deposition (for example,sputtering or CVD), electro-deposition and chemical vapor deposition. Ina particular embodiment of the spring that is suitable for making goodelectrical contacts with contact pads or terminals of various materials,the spring is comprised of a sputter deposited core of about 1-4 μmthick Mo—Cr films with a stress gradient across the thickness(compressive bottom to tensile top). The multilayer thin films, e.g. Nior its alloy overlying the core are deposited on all sides of the core,after the free portion of the spring is released, using film depositiontechniques such as electroplating techniques (dc and/or pulsedeposition), to an overall spring thickness of about 18-35 μm. Theelectrodeposition technique, both electroplating and electroless, is apreferable technique, for coating the core film. Pulse plating, a methodfor electrodeposition, is particularly useful for the coating, as ittends to produce denser films. Composition modulated electrodepositiontechniques can also be used to deposit the multilayer films.

[0063] We have discovered that for stress metal springs, whethercomprised of multilayer of very thin (less than about 2 μm thick) filmsor relatively thicker, e.g. greater than about 2 μm films, with athickness of 1-45 μm for the free portion, are quite suitable for makinggood electrical contacts with various materials comprising electricalcontact pads or electrical terminals on different substrates orelectrical components. A preferable thickness range for fabricating thefree portion of the springs is 4-35 μm. Excellent electrical contacts(very low contact resistance) have been obtained between tips at the endof the free portions of these springs, with an appropriate thickness inthis preferable range, and electrical contact pads or terminals, whichcontain mainly gold (Au), copper (Cu) or commonly used lead-free orlead-bearing solders or aluminum (Al).

[0064] In another equally preferred embodiment, the failure resistanceof the springs, comprised of different films with relatively thin, e.g.0.2 μm or thick films, e.g. 10-15 μm, is substantially increased byelectrodepositing, e.g. by electroplating the films onto the core filmin such a way that all or at least the relatively thick overlying films,specially those near the spring surfaces, remain under a compressivestress. This means that the completed spring is designed to bepre-stressed. In order to maintain the pre-stressed condition, both theoverlying and core material should have high elastic limit that resistsplastic deformation. In addition, the interface between different filmlayers should also be strong. In a typical embodiment, the overlyingfilm with compressive stress is comprised of nickel, with a thickness ofabout 10 μm on each side of a Mo—Cr core film. Use of appropriatedeposition conditions, e.g. additive concentrations in theelectrodeposition bath, produces such a film. The resulting springwithstood many touchdowns without any failure. One of the reasons forspring failure is the development of high tensile stress in the springsurface as it is pressed to make contacts with electrical contact padsor terminals. The fatigue strength of materials is, in general, lowerunder tensile mean stress than under compressive mean stress. Thedescribed solution minimizes the development of tensile stress in springsurfaces when pressed into contact with contact pads or terminals, andthus increases the resistance of the spring to failure. This scheme, andthose to be described in the following paragraph, also allow fabricationand use of thin stress metal film springs with relatively large overallthickness that is useful for generating high contact force needed at theelectrical contact pad or terminal for some applications.

[0065] Note that the completed spring is designed to be pre-stressed.Pre-stressing is preferably achieved by compressive stress. However, therange can be low tensile stress to compressive stress, e.g. tensile 30MPa to compressive 70 MPa. Stresses are different for differentthickness of the Ni plating for the same additive concentration. Forthinner films the stress is higher. For example, for 1.5 μm thick filmthe stress is about 70 MPa compressive. The range of compressive stressthat can be produced in plated Ni springs with the same additiveconcentration is about 6-70 MPa (compressive) for thicknesses rangingfrom 25-1.5 μm. Thus the stresses for various Ni film thicknesses can betailored through variation of additive concentration.

[0066] Another effect of change in additive concentration in the platingsolution is reflected on the grain size of the plated films. For thecurrently plated springs with increased additive concentration, thegrain size was found to be one-fifth (20%) of the samples plated earlierwith smaller additive concentration. The smaller grain size increasesthe yield stress of thin films (d^(−1/2) dependence). This is animportant contributing factor in increasing the lifetime of our springsduring repeated touchdowns. A preferred range of grain diameter in thefilm, e.g. Ni that overlies the spring core is 3-500 nm, and a typicalpreferred value is 50 nm. The plated overlying films seem to be strongeras the grains become more equiaxed, e.g. ratio of larger to smallerdimension of grains is less than 2.

[0067] In another equally preferred embodiment, selection of filmmaterials for deposition of a multilayer stack of films, whethercomprised of relatively thick, e.g. thicker than about 1.5-2 μm, or verythin (less than about 1.5-2 μm) films, is made in such a way that filmswith lower elastic modulus are deposited near the spring surface, andfilms with increasingly higher modulus are deposited towards the core.In a variation of this embodiment, films are selected and deposited overthe core in such a way that it results in a reasonably continuousincrease of the elastic modulus from the spring surface to the springcore, namely in graded deposition in compositions. Graded transition incompositions and elastic modulus from the spring surface to the core,either continuous, or in fine discrete steps, across an interfacebetween two different materials can be used to distribute the stressesat critical locations, and thus suppress the onset of permanent damage.In these configurations, as the springs are pressured to make electricalcontacts with contact pads or terminals, the critical tensile stresses,which results in the nucleation of damage at the surface, are lowered atthe surface as the higher modulus beneath the surface spread thestresses from the surface to the interior of the spring. This reducesthe probability of crack initiation at the spring surface duringrepeated touchdowns, and thus the spring lifetime is increased. Asexamples of this embodiment, the spring surface layers (i.e. outersurfaces of the overlayer stack) consists of palladium alloys (such asthose comprising Ni, Co, or Pt), gold alloys (such as those comprisingNi or Co), Pt alloys etc.; whereas the film layers closer to the springcore, e.g. Mo—Cr, are comprised of nickel or Ni alloys, e.g. Ni—Co. Thehigher the concentration of Pd or Au in nickel is, the smaller is theelastic modulus. Thus, in another illustration, the spring lifetime isincreased by depositing nickel or its alloy with higher elastic constantonto the core film, e.g. Mo—Cr followed by deposition of successivelayers of overlying films that contain Ni with increasing amount of Pd.The outer films in this case contain relatively high concentration ofPd, e.g. 10-50 w% Ni and 90-50% Pd. For graded films, as mentionedabove, the Pd concentration in Ni is changed continuously from the coreto the surface. The latter can be achieved through variation ofdeposition parameters of conventional deposition technique, e.g.electrodeposition, during the deposition process. The core material canbe a material with higher elastic modulus than that of other films.

[0068] Various material combinations can be used to fabricate themultilayer film stack over the stress metal spring core. These areapplicable to both very thin (less than 2 μm) and relatively thick, e.g.2-20 μm, individual constituent films. Such combinations are selectedfrom groups of materials including Ni, Au, Ag, Cu, Co, Rh, Ru, Pt, Os,Pd, TiN, W or their alloys, such as Ni—Co, Pd—Ni, Pd—Co, Co—Pt, Au—Pt,Pd—Rh, Ni—P, Ni—Mo, Ni—Co—Pd, Ni—Mo—W, Ni—P—W and etc. Solid solutionscomprising at least two materials, such as Ni with less than about 12%W, or Ni with 2% Mo, or Cu—Rh—Pd or Pd—Ni or Pd—Co, Ni—Co or Co—Pt andetc. are particularly good candidates to fabricate the multilayer withinfilm stack, as they enhance the mechanical properties of the film.

[0069] Multilayer films are particularly suitable for making electricalcontacts with terminal/contact pads during testing and burn-in processthat requires passage of a relatively high current. A common practice inthe industry is to make occasional probe contacts to terminal pads at acurrent level of 250-500 mA. This often leads to contact failure becauseof excessive heat generated at the contact region. Modeling of heat flowhas shown that the highest temperature is reached near the spring tipregion. Melting of the spring tip regions has also been observed in somecases. It is shown in this invention that addition of a good heatconducting film, such as Cu with a typical thickness of about 0.75-2 μm,in the multilevel stack of films of which the spring is comprised, canovercome the problem. Presence of Cu allows the heat to be dissipatedquickly from the tip region and thereby minimize the damage duringtesting or burn-in. Of course, different thicknesses, e.g. greater than2 μm, of the good heat conducting films will also work for this purpose.

[0070]FIG. 4 is a schematic diagram showing such a spring finger thatincludes a Cu film for improved thermal conductivity. Other high thermalconductivity materials can also be used instead, or in addition to Cu,for improving the heat dissipation from the spring tip. Examples are:Au, Ag, Al etc. In this solution, the films with high thermalconductivity can be deposited before or after the spring is lifted. Ifthe film is electrodeposited after the lifting, the high conductivityfilm can be deposited all around the core film. If the film is to bedeposited on one side of the spring, or only at and near the tip region,the deposition can be done before the spring is lifted and patterned.

[0071] The build up of high temperature near the spring tip region isminimized if the probe-tip to contact pad electrical contact resistanceis minimized. The reduction of the contact resistance can be achieved byincreasing the force applied by the probe-tip onto the contact pad also.We have found that the electrical contact resistance between the springtip and contact pad or terminal is less than 1π when the contact isreliable and stable. A preferable range of values for good electricalcontact and good heat dissipation is about ≦0.1-0.2 Ohm.

[0072] In another embodiment of the invention, the thin film springs canbe strengthened against failure by strengthening the interfaces betweendifferent film layers against defect propagation and by enhancing goodbonding between two adjacent film layers. For example, the interfacebetween the core spring material Mo—Cr and an adjacent layer of Ni filmcan be significantly strengthened, and bonding between the two layers atthe interface be made substantially stronger, by phasing-in Ni at theend of Mo—Cr deposition. The phasing-in can be achieved as follows.Shortly before the end of Mo—Cr deposition, Ni deposition is initiated.Then the Mo—Cr deposition rate is gradually brought to zero, whileadjusting the deposition parameters appropriately increases Nideposition rate. For subsequent Ni or its alloy deposition on the coreby other methods, such as eletrodeposition, Ni or its alloy is depositedon the Ni surface of the core. As a result a robust bond formationoccurs and the interface strength is enhanced. Such interfaceengineering can also be applied to enhance the interface quality betweentwo adjacent electrodeposited film layers. In that case, near the end ofdeposition of an electrodeposited film A, which is to be followed bydeposition of another electrodeposited film B, an alloy of A_(x) B_(1-x)can be deposited using suitable process parameters.

[0073] In another embodiment of this invention involvingelectrodeposition to deposit thin film coating layers on the springcore, the deposition parameters are changed intermittently duringdeposition in order to improve the quality of the coating films. It isknown that electrodeposition of relatively thick films of a materialoften shows increased porosity in the film layers near the top of thefilm typically at a thickness exceeding about 1.5-2 μm. Consequently,changing the film parameters, e.g. dc plating to pulse plating orchanging the current density during deposition, improves the quality ofthe film significantly. As a result, the film becomes stronger andresists early failure during testing or operation. Variation ofdeposition parameters during electrodeposition can change themicrostructures, e.g. grain size, and crystallographic structures of thedeposits, as well as film stresses.

[0074]FIG. 5 is a schematic diagram illustrating a solution for stressmetal spring design and fabrication according to another aspect of theinvention, wherein 501 refers to an electrical pad, 502 to a metalfilled via, 503 to an insulator film, such as a polymer film, 504 to anelectrical trace, 505 to a release layer, 506 a plated film, 507 aspring core, 508 to a plated film at surface, and 509 to a substrate.This design allows establishment of good electrical contacts at reducedforce and thus results in a substantial increase in the resistance tofailure during repeated touchdowns. The fatigue life of a structure is astrong function of the applied stress. Thus, achieving low stablecontact resistance at a low contact force, enabling lower stresses insmaller size structures, is highly desirable for increasing the springlifetime and performance. Some miniaturized springs fabricated in adifferent way from this invention, which are also used for testing orburning-in of electronic components, reportedly require a contact forceranging from 2-150 gf. In several experiments, we have demonstrated thatthe stress metal springs with the basic structure according to thisinvention as shown in FIG. 5 can make a very good contact at a farsmaller force. In these experiments, some of the films, e.g. Ni or Nialloy, overlying the core film on all sides were not very thin, e.g.thicker than 2 μm, and the outer surfaces of the spring are coated witha relatively hard, environmentally stable material, such as Pd—Co or Rh.However, a force as little as 1.4 gf at the contact between thesesprings and Al, one of the most difficult materials to make electricalcontact with, resulted in a good, low and stable contact resistance. Infact, we have found that the force should preferably be maintainedwithin a range of about 0.8 to 10.0 gf for effective electrical contactbetween these springs and Al. A higher force tends to damage the contactpads 501, and a lower force fails to penetrate the surface oxidereproducibly. For contacting other materials, such as Au, Cu, andsolders, which do not form tenacious oxide like that on Al, the forcerequired to make good electrical contacts is significantly smaller, forexample 0.2 gf. We have obtained good electrical contact between ourprobe springs and gold contact pads at a force as low as 0.01 gf. Asmentioned above, establishment of a good electrical contact with lowcontact resistance also allows testing of circuits or devices at ahigher current without encountering significant degradation of springquality due to high heat problem. Consequently, the probe springs withthe structure shown in FIG. 5 are desirable for tests or burn-inrequiring passage of higher current. Similar springs with multilayerstructure comprised of very thin films, less than about 2 μm forexample, are also suitable for such tests or burn-in that requirepassage of higher current.

[0075] Capability to make good electrical contacts between the springsand the contact pads or terminals at a very low force, as indicatedabove, results in a number of benefits. The introduction of coppermetallization and low dielectric constant materials in deep submicronintegrated circuits by the microelectronic industry has opened upsubstantial demand for low force probe contacts during testing andburn-in of the chips. The low k dielectric materials are relativelyfragile. Consequently the spring structures described here areparticularly suitable for applications to circuits comprising Cu filmsand low dielectric constant materials. These springs can make goodelectrical contact on Cu with a relatively low force, for example, lessthan 1 gf. So chances of damaging the circuit elements are minimized.

[0076] Another important benefit from low force contacts is related tothe fabrication of interposers. As is known in the art, probe cardassemblies often use interposers in between the ProbeChip (or spacetransformer) and load-board (PCB that is connected with the tester) toestablish electrical connection between the IC to be tested and thetester. Cantilever type of springs is attached to these interposers forfacilitating electrical connection. For currently available probe cardassemblies in the market, the force applied by each of these interposersprings is relatively high, e.g. 15-30 gf. Interposers havingminiaturized stress metal springs fabricated by the present inventioncan make electrical contacts with opposing contact terminals at a farlower force, e.g. 0.005-2 gf, because the contact terminals aregenerally comprised of materials other than aluminum, e.g. gold. Such asmall contact force can be applied by these springs consisting of onlythe core material, e.g MoCr, without any plating. Of course, relativelythin layers of plating are preferred, e.g. with gold, in someapplications for increasing electrical conductivity of the springs orthe mechanical properties, e.g. wear resistance of the spring tips. As aresult of the low force contact springs, the total force applied by theinterposers having thousands of springs is vastly reduced. Thus, use ofthe present photolithographically patterned miniaturized stress metalsprings for the construction of probe card assemblies includingprobeChips, interposers and assembly fixtures for testing and burn-in,as well as packaging, are greatly simplified by the use of these lowforce springs, as bending, warping and alignment problems are minimized.Because of the low force exerted by the springs of the present inventionon the contact to establish good electrical connection, bulky mechanicalsupports for assembly, and even the interposer, can be dispensed withfor many applications. Thus, the use of the low force springs, describedherein, results in a significant increase in yield and reliability, aswell as reduction of cost and complexity.

[0077] It is known that increasing the thickness of the spring canincrease the spring contact force on contact pads or electricalterminals. Mathematical expressions are available to calculate the forceas a function of spring dimensions. In stress metal springs, the corematerial, e.g. Mo—Cr, thickness is kept typically less than about 5-6 μmto facilitate lifting of the free portion of the spring following itspatterning on the substrate. Films are subsequently deposited onto thespring, e.g. by electrodeposition, to increase its thickness forapplications requiring increased contact force. Deposition of additionalfilms selectively onto the spring using photolithography or othermethods is quite complex and costly due to the non-planar structure ofthe springs. In this invention, a much simpler and effective solutionhas been applied to deposit different films by electrodeposition ontothe spring, as well as on the circuit traces, if needed. This solutiondoes not require the use of any masking. In this case the electricalcontact to the arrays of springs is made from the backside of thesubstrate 509 by blanket deposition of an electrically conducting thinfilm onto it or backside patterning the film to give better currentdensity control. The electrical continuity is established using a via,such as 502, through the substrate, which are filled with electricallyconducting materials that are in electrical contact with the springs,adhesion layer 505, spring metal 507, traces, e.g. 504, or contact pads,e.g. 501. Consequently, films are deposited only onto the electricallyconducting surfaces that are electrically connected to the appropriateterminals of the power supply at the back of the substrate. This schemeallows selective electroplating on all surfaces on the lifted springthus enveloping the spring and also electroplating, traces, and othermetal structures that are not covered with an insulating material.Preferred substrates comprise inorganic materials such as ceramic,quartz, silicon, glass. Other substrates comprising organic materials,such as polymer, epoxy, FR4 and polyimide can also be used within thescope of this invention. An example of the latter group of substratesare printed circuit boards using FR4, Dupont's Thermount and Nelco'sN4000.

[0078] Previous work (WO 01/48870) also reported plating of materialsonto lifted stress metal springs. However, due to the non-planarstructure, they used complicated photoresist patterning to electroplatematerials on one surface of the lifted springs. In our work we havefound that that method does not work well at all for a manufacturingprocess, as the presence of stresses in the electrodeposited filmsprimarily on one surface of the core spring material affects the springlift height. In addition, the photoresist deposited onto the freeportion and the base of the spring tends to pull the lifted portiontowards the base uncontrollably, apparently due to surface tensioneffect, because the spring core is made very thin to allow appropriatelifting. As a result, this method is not suitable for obtainingreproducible and controlled lift height for arrays of springs. In thepresent invention, this problem is eliminated by electrodepositing anenvelope of material over the spring core without the use of anyphotoresist mask as illustrated in FIG. 5. The stresses on the two sidesof the core spring are also reasonably balanced in this case, therebyminimizing the alteration in the spring lift height due to plating.Maskless plating of the spring cores is thus very desirable forproducing an envelope of electroplated films covering all core surfaces(and also other electrically conducting surfaces around the springs, ifdesired) using substrate through-vias to establish electrical contactsfrom the substrate-surface opposite to the surface where the springs arelocated.

[0079]FIG. 6 shows the lifted spring prior to plating and FIG. 7 showsthe spring after plating. To maintain the appropriate stress of theplated film it is important to compensate for changes that occur incurrent density as the area of the lifted springs changes. Currentsupplies must be programmed to manage stress in the film as tocompensate for changes in the spring thickness which reduce currentdensity.

[0080] A problem often encountered upon a large number of touchdownse.g. 100,000, is the build up of contact pad materials on the stressmetal spring tip regions. This affects the contact resistance and thelifetime of the spring, particularly if the contacting pads arecomprised of aluminum. Coating the probe tip region with a metal or anelectrically conducting material to which the contacting metal e.g. Al)does not adhere to well or at all minimizes this problem. Examples ofsuch coating materials are platinum group materials including rhodium(Rh), palladium and ruthenium and their alloys comprising two or moreadditions, e.g. palladium-nickel, palladium-rhodium, palladium-cobalt,palladium-gold-rhodium as well as titanium nitride, Ir—Au, Ir—Pt,gold-cobalt, zirconium nitride etc. Although thin films of such coatingmaterials are deposited onto the body of the probe springs for low forceand lower current applications, after stress metal springs are releasedfrom the substrate, it is desirable to deposit the coating only near thespring tip region for some applications. One reason for not depositing acoating material all over the main body of the spring is to haveflexibility in choosing the coating material, for example, for thedesired elastic modulus, and the film thickness, for selectively coatingthe spring tip region. Presence of some coating materials with arelatively large thickness on the main body of the spring can affect thereliability of the springs. This invention provides a new solution todeposit such a coating very controllably on only the tip region ofstress metal springs using a technique that is compatible with theintegrated circuit technology. In this solution, a “button” comprisedpreferably of a plurality of electrically conducting films is fabricatedat the spring tip region for making contacts with the electrical contactpads or terminals. In one embodiment of this solution, the coatingmaterial, as mentioned above, is deposited as the final overlayer ontothe “button” before the free portion of the probe spring is releasedfrom the substrate. As a result, the problem associated with subsequentlifting of the free portion of the spring to the appropriate height isminimized, because only a small portion of the spring is constrained bythe tip coating material, while the rest is free to bend and lift. Thisapproach is used for lower force springs that do not require additionalthickness for higher force or improved spring conductivity (MoCr springsare thin and resistive).

[0081] The process steps for fabricating springs with “buttoned” tips,as mentioned above, are as follows. Following the deposition of thestress metal spring core film, e.g. Mo—Cr, a mask, e.g. photoresist, isdeposited onto the core film and patterned using techniques, e.g.photolithography, to define a spring. The spring is etched, thephotoresist removed, and an additional photo process is followed, suchthat all of the core film, except the spring tip regions, remainscovered with the mask. Subsequently the film, e.g. Rh, that is to belater deposited as overlayers onto the core of the spring is depositedto the desired thickness onto the exposed spring tip regions followed bydeposition of appropriately thick, e.g. 1-4 μm, final overlayercomprising the coating material mentioned above, e.g. Pd—Ni, Pd—Rh,Pd—Co, Rh or TiN. The coating layer thickness range could be of coursebe higher also, e.g. 1 to 20 μm, for this invention to work. Note thatIn a variation of this embodiment, films to be deposited onto the springtip region can also be comprised of materials other than the one that isto be later deposited onto the main body of the spring. Upon removal ofthe mask, etching is used to undercut the spring and release the freeportion of the spring from the substrate. This is followed by depositionof overlying films to the desired thickness onto the main body of thesprings, while the spring tip regions already fabricated is keptprotected with a mask of, for example, photoresist or polyimide.

[0082] The thickness of the resulting tip region could be designed to beapproximately equal to the remaining part of the lifted springs.Subsequently the mask is removed and the probe springs with the desiredthickness of the coating materials on the spring tip regions areobtained. Although a number of film deposition techniques can be used todeposit the overlayers and the final coating layers, electrodepositionis a preferred for such depositions. In another variation of thisembodiment, selective deposition of overlying films onto the spring tipregions prior to spring release can also be done after patterning thedeposited core film into spring fingers, instead of patterning thespring fingers after the coating films are deposited onto the spring tipregions. The rest of the subsequent process steps are the same for bothembodiments.

[0083] The preferred method for fabricating buttons on raised platedsprings like the one shown in FIG. 7 is described below. The spring tipregion is selectively coated (button fabrication) with one or moresuitable materials, after the spring is lifted from the substrate, usingphotolithography. In this method, a photoresist is deposited usingwell-known techniques, such as spinning or spraying or plating, onto thelifted springs. The preferred method is to spin on photo resist. Unlikenon-plated springs, thick photo resist can be applied to the springsbecause the relatively thick envelope of material over the core stiffensthem substantially. Because of this enhanced stiffness the spring heightis not affected significantly by the application of photo resist. Thespring covered in photo resist is shown in FIG. 8. For plating of thephotoresist, the electrical terminals at the back side of the substrateis used for connection to the power supply, as discussed above. Thebackside terminals are connected to the springs on the front side of thesubstrate through metallized vias. The photoresist is then selectivelyremoved from the spring tip regions, including the top surface and thesidewalls of the tip regions, using photomask and photolithographytechniques as shown in FIG. 9. Subsequently the tip coating material, asdescribed in the preceding paragraph, e.g. Pd—Ni, Pd—Co, is depositedonto the spring tip regions using conventional techniques, preferablyelectroplating. Sputtering or CVD can also be used in which case,coating materials are also deposited onto the photoresist layer, whichare subsequently removed along with any unwanted overlying coatingmaterials using conventional solvents, leaving the coating material onthe tip regions only. Electroplating of the spring tip regions notcovered with photoresist allows substantial coverage of the tip region.The preferred materials for buttons are comprised of platinum groupmaterials (namely Pd, Pt, Rh, Os, Ru and Ir), Ni, Co, Au and Ag. Thisstructure is shown in FIG. 10.

[0084] In the process described in the preceding paragraph, the tipbuttons are plated following the deposition of the relatively thickenvelop of a material e.g. Ni) onto the core, which stiffens the springsubstantially. Because of this enhanced stiffness and the relativelysmall area coverage of the spring by the button, spring lift height isnot affected significantly by button plating.

[0085] After the Button tips have been plated, the photo resist isremoved leaving the final structure as shown in FIG. 11. Note the anchorportion 516.

[0086] In case metallized through vias are not present in the substratefor enabling backside connection, a variation of the solution describedin the preceding paragraph can also be used to selectively apply coatingto the spring tip region following the lifting of the spring core filmand deposition of the overlying films. In this case, an electricallyconducting material, such as Au, Ag or Cu, is first blanket deposited,using a technique such as sputtering or electrodeposition or CVD, allover the substrate containing the stress metal springs after the liftedspring is fabricated to the desired thickness including the overlyingfilms over the core films. This conducting layer is used for providingelectrical connection for button electroplating of the spring tips. Thenphotoresist is deposited all over the electrically conductive surfaces.Using photolithography techniques, as described in the precedingparagraph, the coating material is selectively deposited onto the springtip regions only. The thin conducting material deposited prior to thephotoresist deposition is then removed by wet or dry etching techniques.

[0087]FIG. 12b illustrates a particular embodiment of the selectivelycoated spring tip region with an improved spring lifetime, wherein 1215refers to a tip button with protective coating, such as Pd—Co or Pd—Nialloy, etc. Here the free portion 1218 of the spring is substantiallytapered. As discussed later, for this embodiment, the spring lifetime issubstantially enhanced, and the coating material at the tip region doesnot show any significant degradation, during repeated touchdowns.

[0088]FIG. 13 is a drawing showing the result of tip (button) platingusing photoresist application followed by patterning to expose the tipregion and selective coating of the tip region by electroplating of aPd—Co alloy. All the tips can be substantially covered with the platedbuttons, although generally only a relatively small area of the springtip contacts the IC terminals or electrical contact pads on othercomponents of probe card test assemblies for electrical testing orburn-in operations. The large area coverage of the spring tips by thebutton materials provides flexibility in designing the springs and testassembly. In addition, joining of the springs to IC terminals or contactpads of electrical components for packaging applications usingtechniques such as soldering is greatly facilitated through the use ofbuttons that substantially covers the spring tips. In such cases, buttonplating materials are selected from the group that form good reliablebond with the solder e.g. Sn containing alloys, Pb—Sn or Pb-freesolders) commonly used in microelectronic packaging industry. Examplesof button materials or spring coating materials for making contacts withsolders or conductive adhesives in packaging applications aremultillayer stack of films comprising platinum group materials, e.g.palladium, platinum, ruthenium etc., as well as cobalt, nickel, gold,copper, cobalt or alloys.

[0089] The resistance of the stress metal spring to failure can also beincreased by designing springs with varying width that increases fromthe tip area to the base of the finger. Most of the spring fractureduring repeated touchdowns occurs near the base of the springs. Becausethe stress generated during contact with contact pads is, in general,the highest near the base of the spring finger, the stress near the basecan be reduced substantially by increasing the width near the base area.For example, the free portion of the spring can be patterned to have asubstantially trapezoidal shape. Similar increase in the failureresistance can also be achieved by making the region nearer to thespring-base thicker which, for constant applied force, also reduces thestress in the proximity of the spring-base region.

[0090]FIGS. 12a and 12 b are schematic diagrams showing a particularembodiment 1200 of the springs with varying width in tapered shape,wherein 1216 refers to a fixed spring base and 1218 refers to a freeportion of spring with tapering for relatively uniform stressdistribution. Having the free portion 1218 of the spring tapered resultsin a significant increase in the failure resistance of the spring. Thekey point here is to shape the free portion 1218 of the springappropriately, in this case through tapering, so that the bending stressis evenly distributed along the spring 1200. In addition, the springcompliance is increased because of tapering. This concept thus allows adesign solution to maximize force at a minimum stress for a givencompliance range. Note that the parallel sides at the base region (i.e.,the anchor portion) can also extend into the lifted region (i.e., thefree portion) to some extent, such as 1218 a, before tapering begins.FIG. 12b schematically shows a buttoned and tapered spring which havebeen found to withstand large number of touchdowns without fracture.

[0091] In exemplary embodiments, stress metal spring core members,comprising a free portion and an anchor portion attached to thesubstrate, are materials with high elastic modulus, such as Mo, Mo—Cr,W, Ti—W. The core member is selectively coated, after the free portionof the spring is lifted, to cover all its exposed surfaces. The resultis an envelope comprising at least one metal films deposited byelectroplating without mask using metallized through hole vias in thesubstrate to establish electrical contact from the backside (opposite tothe spring side) of the substrate. The envelope balances the stress inthe free portion, and extends to the anchor portion without anydiscontinuity which mechanically weakens the film causing earlyfracture. Typically, Ni or Ni alloy is deposited onto the core member.Additional film, such as Pd alloy film is optionally electroplated ontoNi, it needed. Selective deposition of additional layer of the palladiumalloy film onto the spring tip region is carried out using conventionalphotolithography and deposition techniques, such as electro-deposition(electroplating and/or electroless) or sputtering or CVD. Typicalthickness of Mo—Cr is 4 μm. The thickness of electroplated nickel andpalladium alloy films on each side of Mo—Cr film are 2-20 and 1-10 μm,typically 12 and 4 μm, respectively. In this case the elastic modulus ofthe films decreases from the core towards both surfaces of the spring.The thickness of the button comprising additional deposit of palladiumalloy film, for example, at the contact tip region is 120 μm with atypical value of 12 μm.

[0092] Another aspect of this invention is to eliminate points of stressconcentration on the spring surface. We have observed that often thespring failure, such as crack, is initiated at the surface duringrepeated touchdowns. Thus, surface roughness needs to be minimized. Muchof the roughness on the sidewall of the lifted spring, as shown in FIG.5, originates during patterning the core film, such as those composed ofMo—Cr, W or Zr—Ni for example, by wet etching. Overlying films, such as506, subsequently deposited on the core 507 follow the rough contour ofthe sides, resulting in a rough surface on the side of the completedspring structure. According to this invention, forming the spring corepattern by dry etching involving ionized species minimizes thisroughness. In using electroplating to build up the overlying films, theroughness is also minimized using the process of sequential plating andreverse plating (deplating) to build up the spring thickness. Reverseplating parameters are adjusted so that only a fraction of the platedthickness is removed during reverse plating. Polishing the sides of thewet etched core 507 initially or of the fully plated springs byelectropolishing, chemical or electrochemical polishing can alsominimize the roughness.

[0093] In another embodiment, stand-offs are provided on the substrateor electrical components so that the springs, in which the core film iscovered everywhere with the overlying film deposits, are constrained toa maximum overdrive on the electrical contact pads or terminals that isallowed by the designed heights and locations of the stand-offs.

[0094] The solutions described above can also be used in themanufacturing of various other cantilever springs, which are not stressmetal springs that are partially lifted as a result of the presence ofan intrinsic stress gradient in the film. One of the major concernsabout the performance of these other cantilever springs is also thepropensity to failure, e.g. deformation or crack formation, near thebase or the anchored end of the cantilever springs, as the stress inthis region is the highest when the spring tip-ends are pressed intocontact with the contact pads, i.e. input/output (I/O) pads of the waferor other substrates or components of the test or burn-in assembly.Mathematical expressions are available to show the effect of springlength on the stress near the base region. In order to minimize thestress at the base region during flexing of the spring as it ispressured into contact with the contact pads, and thus increase itsresistance to failure during repeated touchdowns, the length of thespring is currently designed to be relatively large, e.g. approximately700-2,000 μm. However, this limits the applicability of cantileversprings for testing and burn-in of some of the current and futuregenerations of ultra-miniaturized integrated circuits, for which thespring probe arrays should match the highly dense arrays of device I/Opads with tighter pitches, e.g. approximately 20-50 μm. Consequently, itis very desirable to find means to make shorter springs with tighterpitches that are strong enough, particularly near the base region towithstand higher stresses without failure.

[0095] The need for increased spring constant for applying requiredforce at the point of spring contacts to the contact pads makes itnecessary that the free portion of the cantilever type of springs bemade thicker. In some embodiments, springs of higher thickness are madeby electroplating one or more metals or their alloys, such as nickel ornickel alloys, or palladium alloys, on photo-lithographically patternedfree standing spring core such as Mo—Cr alloy. In some otherembodiments, the springs are patterned, using photo-lithography, andfabricated by electroplating relatively thick layers of at least onemetal or metal alloy films, e.g. nickel or nickel alloys, on seedlayers. In many of these embodiments, button type contact structures arealso provided at the contact tip regions to improve the contactproperties and maintain the contact integrity during repeated touchdownsat the time of wafer testing and burn-in operations. However, suchembodiments still require relatively thick films for constructing themain body of the springs in order to apply the required contact force atthe contact tip end. For relatively shorter springs, which areapproximately 100-700 μm long, the increased spring thickness results inhigher stresses near the base end, resulting in a lower spring lifetime.

[0096] Described below are solutions for manufacturing shortercantilever type of springs with or without buttons like contactstructures at the spring tip regions, in which the strength at thesprings' base or main body regions is strengthened against mechanicalfailure, resulting in a significant enhancement of the performance,strength, durability, and lifetime of such springs.

[0097]FIGS. 14a and 14 b illustrate two cross-sectional views of atypical freestanding non-stress metal cantilever spring according to oneembodiment of the invention. The freestanding cantilever springcomprises a base region 1401 at one end that is attached to anelectrical contact pad 1402 of a substrate 1403, a contact tip region1404 at the other end of the spring, a button 1406 together with thecontact tip region 1404, and a main body deposited with a Ni film 1408and a Pd-alloy film 1409. The spring length can be substantiallyparallel to the surface of the substrate it is attached to, or it canextend away from the substrate surface making an oblique angle to thesurface. Typically the base 1401, the tip 1404 and the main body of thespring are fabricated with the same materials in the same operation, forexample, by using thin film deposition techniques such aselectroplating, sputtering, or CVD.

[0098] The contact tip region 1404 comprises a button type contactstructures 1406 for facilitating reliable and durable contacts, whichmay be fabricated by selectively depositing films on the contact tipregion 1404 as an integral part of the tip region or separatelyfabricated and affixed to the tip region. Similarly, the base region1401 may be attached to a post that may be integrally fabricated withthe spring, or fabricated separately and joined to the base usingconventional techniques, such as soldering, brazing and etc. For theintegral fabrication of the posts, films may be selectively deposited,using techniques such as electroplating, into holes within sacrificialsubstrates, followed by polishing.

[0099] The presence of button type contact structures 806 at the springtip regions 1404 is useful for achieving reliable and durable electricalcontacts to the opposing contact pads in a wafer test or burn-inassembly. In that case, appropriate materials with desirable contactcharacteristics and thickness can be selected for constructing suchbuttons which are not necessarily, but can also comprise, the samematerials that comprise the main body 1405 or base 1401 of the spring.However, selections of the materials for each of the three parts must besuch that they impart robustness to all parts of the springs allowingthem to withstand the wafer test and burn-in process including repeatedtouchdowns without failure. Many materials suited for variouselectroplating applications have been used in making the cantilever typeof springs. Such materials include, for example, nickel and its alloys,gold, rhodium, Pd and its alloys, copper, elements of platinum group andtheir alloys, titanium, molybdenum and their alloys etc. However,challenges to make shorter springs with the required robustness stillremain. Non-stress metal cantilever springs manufactured today are stillrelatively long, for example 1-2 mm. A major objective in the field isto find a means to fabricate arrays of much shorter and robust springsto support the continuing drive of the microelectronic industry toproduce deep sub-micron integrated circuits with greater circuit densityand concomitant smaller pitch between input/output terminals.

[0100] This invention makes it possible that arrays of such robustcontact springs are fabricated by applying specific material selectionprinciples for the construction of the buttoned or non-buttoned springs,which comprises metal films. Selection of the appropriate materials byapplying these principles provides particular methods of film depositionand thus results in the fabrication of contact springs with the desiredrobustness.

[0101] A specific material selection principle that has been found tohave a highly significant effect in improving the performance andreliability of the springs is as follows. The multilayer filmscomprising the three parts, i.e., a base, a tip region, and a body, ofthe spring and the button should have a graded material composition sothat the films with lower elastic modulus are deposited near the springsurface that makes contacts with the IC terminals for testing, and filmswith increasingly higher modulus are deposited towards the oppositesurface. The mechanical strength of the button is not as critical afactor as the mechanical strength of the main body and base region ofthe springs for determining the robustness of the springs. However,based on the teachings of this invention, the button films can also beoptionally selected and deposited, if needed, in such a way that themodulus of the film at the button surface has lower elastic modulus thanthe underlying film layers, the modulus of which increases progressivelyaway from the button surface. Such a graded transition in compositionsand elastic modulus from the spring contact surface to the oppositesurface, either continuously or in discrete steps across an interfacebetween two different materials can be used to distribute the stressesat critical locations, and thus suppress damages to the spring. As aresult the lifetime of springs increases. This increases the resistanceto mechanical failure in the spring everywhere including the base 801 ofthe spring as the spring tips 804 are pressed into contact with thecontact pads on another substrate, such as semiconductor wafers or othercomponents of a test or burn-in assembly.

[0102] According to the above principle, one exemplary embodiment of thenon-stress metal cantilever springs comprises nickel film as a baselayer with palladium-approximately 20% cobalt or palladium-approximately20% nickel alloy film as the overlayers, because nickel has a higherelastic modulus than that of the palladium alloys. Other films may alsobe deposited to form the multilayer springs, as long as the selectionprinciple is applied, in general, to determine the deposition sequence.Additionally, very thin film layers may also be deposited in between twomajor film layers, as needed, to improve the interface strength oradhesion. For example, a gold or nickel or rhodium strike can be usedfor this purpose, as is well known to persons skilled in the art. Inthis case, the button 1406 on the tip region 1404 can compriseadditional layers of films of the said palladium alloy. The button 1406may be fabricated as an integral part of the contact tip region 1404 orseparately affixed to the tip region 1404.

[0103] Such springs as illustrated in FIGS. 14a and 14 b are typicallydeposited on sacrificial layers, which are subsequently removed toprovide the freestanding cantilever springs. The substrate 1403 may alsohave multilayer metallization and an electrically conducting blind viasor through-vias, as 502 shown in FIG. 5. The films 1408 and 1409 andother additional layers are deposited by conventional techniques such aselectroplating. A suitable thin adhesion promoting layer and/or a seedlayer, comprising materials such as titanium, may also be depositedprior to the deposition of the electroplated layer, as needed. Thethickness of the respective film layers is determined by the desiredcontact force or spring constant, which can be calculated from variousmathematical expressions. Various spring dimensions can be used, 1-50 μmas thickness range for example, based on design requirement, forexample, regarding force and pitch. Taking an overall spring thicknessof 30 μm as an illustration, the nickel and the palladium alloythicknesses in the present embodiment can be 25 μm and 5 μmrespectively. Thickness of the additional layer of palladium alloy inthe button in this case can be 3-20 μm. It is to be appreciated that theabove numbers are used for examples only. A wide variation in thenumbers works well for ensuring the robustness of the springs, as longas the basic principle is satisfied.

[0104] In another exemplary embodiment, the core film is made ofmolybdenum-chromium alloy or titanium-tungsten or molybdenum-tungsten,with sequentially deposited overlayers of nickel and palladium alloyfilms. In this case, the button 1406 on the tip region 1404 comprises,as above, additional deposited thickness of the said palladium alloy.The button 1406 may be fabricated by selectively depositing, usingphotolithography, the additional thickness of palladium alloy film ontothe tip region 1404. The discussion on exemplary thickness range,deposition techniques, adhesion promoting layer and seed layer and etc.,in the preceding paragraph also applies to this case.

[0105] As discussed earlier, the films of the above embodiments ofnon-stress metal cantilever springs can be preferentially deposited withcompressive stress to further improve robustness. The robustness is alsofurther improved by selecting the film deposition parametersappropriately so that the grain size of the films are very small, e.g.3-500 nm. Examples of such deposition parameters include, for example,additive concentration in the electroplating bath, current density, andtemperature.

[0106] The disclosed interconnection apparatus and associatedfabrication methods are suitable for various applications, including butnot limited to testing of electronic components, wafer level burn-in andpackaging of electronic devices. The said electronic components comprisedevices such as integrated circuits, liquid crystal displays, MEMS, aswell as printed circuit boards, or any combinations thereof. Packagingincludes joining and establishment of electrical connections between twocomponents or substrates, using the disclosed contact spring elements,where joining may be accomplished with or without the use of solders orconductive adhesives.

[0107] Abbreviations for Metric Terms and Chemical Elements:

[0108] μm—micron=10⁻⁶ meter;

[0109] nm—nanometer, or millimicron=10⁻⁹ meter;

[0110] Ag—Silver;

[0111] Al—Aluminum;

[0112] Au—Gold;

[0113] Co—Cobalt;

[0114] Cr—Chromium;

[0115] Cu—Copper;

[0116] Mo—Molybdenum;

[0117] Ni—Nickel;

[0118] Pb—Lead;

[0119] Pd—Palladium;

[0120] Pt—Platinum;

[0121] Rh—Rhodium;

[0122] Ru—Ruthenium;

[0123] Sn—Tin;

[0124] Ti—Titanium;

[0125] W—Tungsten.

[0126] This invention applies to all type of miniaturized springs. Thepreferred embodiments disclosed herein have been described andillustrated by way of example only, and not by way of limitation. Othermodifications and variations to the invention will be apparent to thoseskilled in the art from the foregoing detailed disclosure. While onlycertain embodiments of the invention have been specifically describedherein, it will be apparent that numerous modifications may be madethereto without departing from the spirit and scope of the invention.

[0127] Accordingly, the invention should only be limited by the claimsincluded below.

1. An interconnection apparatus for establishing electrical contactbetween two components, comprising. at least one elastic core member,said core member comprising an anchor portion attached to a substratehaving at least one metallized through-via therein filled withelectrically conducting material, and a free portion, initially attachedto said substrate, which, upon release, extends away from said substratedue to an inherent stress gradient in the core; wherein said core memberis electrodepositedly enveloped with at least one layer covering allexposed surfaces of said core member.
 2. The interconnection element ofclaim 1, wherein said envelope comprises electroplated films.
 3. Theinterconnection apparatus of claim 1, wherein said free portion issubstantially tapered having a width that gradually decreases towardsthe probe tip over a substantial length of the free portion.
 4. Theinterconnection apparatus of claim 1, wherein said free portion issubstantially trapezoidal in shape.
 5. The interconnection apparatus ofclaim 1, wherein said at least one layer is an electrical conductor. 6.The interconnection apparatus of claim 1, wherein said at least onelayer is selected from the group of materials comprising any of nickel,platinum group materials which comprise any of palladium, platinumrhodium, ruthenium, osmium, iridium, and, gold, silver, copper, cobalt,tungsten, and any of their alloys
 7. The interconnection apparatus ofclaim 1, wherein the average grain size of at least one of the said atleast one electrodeposited film ranges from 3 to 500 nm.
 8. Theinterconnection apparatus of claim 1, wherein at least one of said atleast one layer remains under compressive stress.
 9. The interconnectionapparatus of claim 1, wherein the grains of at least one of the said atleast one layer are substantially equiaxed with the average ratio oflong to short dimensions being about 2 or less.
 10. The interconnectionapparatus of claim 1, wherein at least one layer near the surface of theelectrodepositedly enveloped core member has a lower elastic modulusthan the said core that it surrounds.
 11. The interconnection apparatusof claim 1, wherein said substrate comprises any of ceramic, glass,silicon, quartz and organic materials.
 12. The interconnection apparatusof claim 1, wherein said core member comprises any of Mo, Cr, Ti, W, Zr,Ti—W, and Mo—Cr.
 13. The interconnection apparatus of claim 1, whereinthe envelope comprises plurality of different and sequentiallyelectrodeposited films.
 14. The interconnection apparatus of claim 13,wherein the electrodeposited films are deposited in such a manner thatthe elastic modulus of the deposited films generally decreaseprogressively from the innermost core toward an outermost surface. 15.The interconnection apparatus of claim 13, wherein the elastic modulusof said electrodeposited films decreases substantially discreetly fromthe innermost core toward an outermost surface.
 16. The interconnectionapparatus of claim 1, further comprising: a film layer selectivelydispensed at a probe tip area onto said electrodepositedly envelopedcore member.
 17. The claim of 16, wherein the film layer is deposited byelectroplating.
 18. The claim of 17, wherein the film layer comprises atleast any one of palladium, rhodium, platinum, iridium, osmium,ruthenium, and cobalt, nickel, gold, copper, silver, and their alloys19. The interconnection apparatus of claim 1, wherein the thickness ofthe electrodepositedly enveloped free portion ranges from 1 to 45microns.
 20. The interconnection apparatus of claim 1, wherein thethickness of each of said at least one layer ranges from 0.1 to 20microns.
 21. The interconnection apparatus of claim 1, wherein said freeportion has a size ranging from 10 μm to 1000 μm in length, 3 μm to 500μm in width, and 0.1 μm to 40 μm in thickness.
 22. The interconnectionapparatus of claim 1, wherein the outermost layer of said at least onelayer comprises any of copper, gold, nickel, and platinum groupmaterials.
 23. A method for manufacturing a plurality of miniaturizedsprings on a substrate, said miniaturized springs each comprising anelectrically conducting core member, said core member having an anchorportion and a free portion, initially attached to the substrate, whichextends away from the substrate upon release due to an inherent stressgradient in the core, said free portion having a tip area at the end,said anchor portion being fixed to a substrate comprising plurality ofmetallized through-vias, the method comprising the steps of.electroplating of spring core members with at least one film layer tocover all surfaces of said core member including free portion withoutusing a mask; and said electroplating of core members is performed usingthrough-vias in said substrate to establish electrical contact to saidcore members from the substrate side opposite to the side where coremembers are located.
 24. The method of claim 23, wherein at least onefilm layer is electroplated with intrinsic compressive stress.
 25. Themethod of claim 23, wherein at least one electroplated film layer isdeposited with an average grain size in the range of 3 to 500 nm. 26.The method of claim 25, wherein the grain size of at least oneelectroplated film is controlled by altering the additive composition inthe electroplating bath, and/or the current density during plating. 27.The method of claim 23, wherein a material used for an inner layer has ahigher elastic modulus; and wherein a material used for outer layers hasa lower elastic modulus.
 28. The method of claim 27, wherein the elasticmodulus of said layers decreases progressively from an innermost layertoward an outermost layer.
 29. The method of claim 27, wherein theelastic modulus of said layers decreases discretely from an innermostlayer toward an outermost layer.
 30. The method of claim 23, whereinsaid at least one film layer is selected from the group of materials,which comprise any of Pt, Pd, Rh, Ir, Ru, Os, and cobalt, nickel, gold,silver, copper, aluminum, tungsten; and an alloy comprising at least anyone of the group consisting of Co, Ni, Au, Cu, Ag, Al, Pt, Pd, Rh, Ir,Ru, Os, W.
 31. The method of claim 23, wherein said substrate comprisesany of ceramic, glass, silicon, quartz and organic materials.
 32. Themethod of claim 23, wherein said core member comprises any of Mo, Cr, W,Ti, Zr, Ti—W, and Mo—Cr.
 33. The method of claim 23, further comprisingthe step of. selectively coating said tip area to form a contact buttonsubsequent to said electroplating of core members; wherein said contactbutton comprises at least one electrically conducting material that doesnot have strong adherence to an opposite contact pad or terminal. 34.The method of claim 33, wherein said tip area is selectively coated toform said contact button before said free portion is released from saidsubstrate.
 35. The method of claim 33, wherein said tip area isselectively coated to form said contact button after said free portionis released from said substrate,
 36. The method of claim 33, whereinsaid at least one electrically conducting material comprises at leastany one of the group consisting of Pt, Pd, Rh, Ir, Ru, Os, and cobalt,nickel, gold, silver, copper, and their alloys.
 37. The method of claim23, further comprising the step of. forming said core film's pattern bydry etching.
 38. The method of claim 23, further comprising the step of.polishing said core film before deposition of said layer.
 39. The methodof claim 23, further comprising the step of. polishing the outermostsurface using any of an eletropolishing, chemical polishing, andelectrochemical polishing process.
 40. An interconnection apparatus forestablishing plurality of electrical contacts between two components,comprising: an elastic core member, said core member comprising ananchor portion attached to a substrate with multilevel metallizationcomprising metallized vias and a free portion, initially attached tosaid substrate, which, upon release, extends away from said substratedue to an inherent stress gradient in the core; wherein said core memberis electrodepositedly enveloped with at least one layer covering allexposed surfaces of said core member.
 41. The claim of 40, wherein saidsubstrate comprises any of ceramic, glass, quartz, silicon, and organicmaterial.
 42. The claim of 40, wherein said core member comprises any ofMo, Cr, W, Ti, Zr, Mo—Cr, and Ti—W.